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arXiv:1807.03643 · Physical Review Applied

Deep Three-Dimensional Solid-State Qubit Arrays with Long-Lived Spin Coherence

C. J. Stephen, B. L. Green, Y. N. D. Lekhai, L. Weng, P. Hill, S. Johnson, Angelo Frangeskou, Phil Diggle, Y.-C. Chen, M. J. Strain, E. Gu, M. E. Newton, J. M. Smith, P. S. Salter, Gavin W. Morley

Published Dec 3, 2019• 46 citations
arXivHTMLPDF

Abstract

The nitrogen-vacancy center (NVC) in diamond is an exciting candidate system for building a quantum computer, because of its long coherence time at the relatively high temperature of 5 K. Scaling up to a useful quantum computer requires having an array of many individual NVCs in one diamond; such arrays can be created, but suffer from much shorter coherence times than in natural NVCs. This study uses laser writing to create arrays of single NVCs with ``natural'' spin coherence times, at the right depth for use with optical cavities, enabling arrays of 200,000 coherent NVCs per diamond.

DOI
10.1103/physrevapplied.12.064005

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Laser-written nitrogen-vacancy centres in diamond membranes for quantum computing

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Manipulation of quantum effects for computing is an important step in the creation of faster and more powerful computational tools. The nitrogen vacancy centre (NVC) in diamond, when in the negative charge state, has shown significant promise towards being used as a qubit, such as optical polarization and readout, long coherence times (T∗ 2 and T2), and a long relaxation time T1. Such times are desired for producing good quantum memory, utilising the spin state of the sites. Cryogenic conditions are required for facilitating the optical entanglement of two NVC, and can serve to significantly extend these times. These sites have been created using a laser writing technique, capable of producing a large number of NVC with good coherence characteristics at arbitrary locations within diamond samples. This thesis reports on methods to reliably create NVC with good coherence characteristics for quantum computing applications by altering the annealing regime used. This annealing regime has been improved by the design and construction of a new vacuum furnace with better temperature control and thermometry. Steps towards the ideal combination of annealing conditions for a range of desired NV concentrations and characteristics have been undertaken, showing good conversion of laser written sites to NVC. NVC have been systematically tracked using a confocal fluorescence microscope apparatus throughout multiple annealing steps, with the collection of coherence data including T1, T2 and T∗ 2 on the same NV centre at multiple annealing temperatures, for the first time. This data allows for tracking of the influence of annealing temperature on the coherence properties of sites without the caveat of comparing data from either different sites, or different diamonds altogether. Such centres show coherence times above 800 μs, with production rates at and above 40% for single emitters. NVC are limited by their weak emission into the zero phonon line. These issues can be alleviated by increasing the collection efficiency of photons, which requires thin diamonds capable of being used with cavity systems. Steps towards producing such diamonds by a thin-region design have been undertaken and show promising results. Further efforts include the use of nanostructures on the diamond surface, which has been implemented and detailed. Magnetic control of NVC within diamond, especially at cryogenic temperatures, introduces some operational complexity to potential quantum operations. By implementing the use of diamond cut at the (111) crystal plane, this alignment is likely to become more straightforward, enabling future work with diamond at this crystal plane. Diamonds have been produced, with NVC laser written in, for the first time.

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The nitrogen-vacancy center (NVC) in diamond is an exciting candidate system for building a quantum computer, because of its long coherence time at the relatively high temperature of 5 K. Scaling up to a useful quantum computer requires having an array of many individual NVCs in one diamond; such arrays can be created, but suffer from much shorter coherence times than in natural NVCs. This study uses laser writing to create arrays of single NVCs with ``natural'' spin coherence times, at the right depth for use with optical cavities, enabling arrays of 200,000 coherent NVCs per diamond.
Manipulation of quantum effects for computing is an important step in the creation of faster and more powerful computational tools. The nitrogen vacancy centre (NVC) in diamond, when in the negative charge state, has shown significant promise towards being used as a qubit, such as optical polarization and readout, long coherence times (T∗ 2 and T2), and a long relaxation time T1. Such times are desired for producing good quantum memory, utilising the spin state of the sites. Cryogenic conditions are required for facilitating the optical entanglement of two NVC, and can serve to significantly extend these times. These sites have been created using a laser writing technique, capable of producing a large number of NVC with good coherence characteristics at arbitrary locations within diamond samples. This thesis reports on methods to reliably create NVC with good coherence characteristics for quantum computing applications by altering the annealing regime used. This annealing regime has been improved by the design and construction of a new vacuum furnace with better temperature control and thermometry. Steps towards the ideal combination of annealing conditions for a range of desired NV concentrations and characteristics have been undertaken, showing good conversion of laser written sites to NVC. NVC have been systematically tracked using a confocal fluorescence microscope apparatus throughout multiple annealing steps, with the collection of coherence data including T1, T2 and T∗ 2 on the same NV centre at multiple annealing temperatures, for the first time. This data allows for tracking of the influence of annealing temperature on the coherence properties of sites without the caveat of comparing data from either different sites, or different diamonds altogether. Such centres show coherence times above 800 μs, with production rates at and above 40% for single emitters. NVC are limited by their weak emission into the zero phonon line. These issues can be alleviated by increasing the collection efficiency of photons, which requires thin diamonds capable of being used with cavity systems. Steps towards producing such diamonds by a thin-region design have been undertaken and show promising results. Further efforts include the use of nanostructures on the diamond surface, which has been implemented and detailed. Magnetic control of NVC within diamond, especially at cryogenic temperatures, introduces some operational complexity to potential quantum operations. By implementing the use of diamond cut at the (111) crystal plane, this alignment is likely to become more straightforward, enabling future work with diamond at this crystal plane. Diamonds have been produced, with NVC laser written in, for the first time.
Diamond's nitrogen vacancy (NV) center is an optically active defect with long spin coherence times, showing great potential for both efficient nanoscale magnetometry and quantum information processing schemes. Recently, both the formation of buried 3D optical waveguides and high-quality single NVs in diamond were demonstrated using the versatile femtosecond laser-writing technique. However, until now, combining these technologies has been an outstanding challenge. In this Letter, we fabricate laser-written photonic waveguides in quantum grade diamond which are aligned to within micron resolution to single laser-written NVs, enabling an integrated platform providing deterministically positioned waveguide-coupled NVs. This fabrication technology opens the way toward on-chip optical routing of single photons between NVs and optically integrated spin-based sensing.
One of the biggest challenges to implement quantum protocols and quantum information processing (QIP) is achieving long coherence times, usually requiring systems at ultra-low temperatures. The nitrogen-vacancy (NV) center in diamond is a promising alternative to this problem. Due to its spin properties, easy manipulation, and the possibility of doing optical state initialization and readout, it quickly became one of the best solid-state spin systems for QIP at room temperature. Here, we present the characterization of the spin-coherence of an ensemble of NV centers in an engineered sample of ultrapure diamond as a testbed for quantum protocols for quantum metrology.
Introduction Nitrogen-vacancy (NV) centers in diamond are widely recognized as highly promising solid-state quantum sensors due to their long room temperature coherence times and atomic-scale size, which enable exceptional sensitivity and nanoscale spatial resolution under ambient conditions. Ultrafast laser writing has demonstrated the deterministic spatial control of individual NV− centers, however, the resulting random orientation of the defect axis limits the magnetic field sensitivity and signal contrast. Methods We developed an all-optical approach for reorienting laser-written NV− centers to lie along a specific crystallographic axis using femtosecond laser annealing. The orientation is determined by polarization analysis, and the annealing and subsequent polarization analysis are repeated until the desired orientation is observed. Results Our method achieves deterministic alignment of NV− centers along the optical axis in (111)-oriented diamond substrates and allows selection between two observable orientation classes in (100)-oriented substrates. The reorientation preserves spatial ordering while producing uniform orientation across arrays of NV− centers. Discussion This approach enables scalable fabrication of orientation-controlled NV− arrays, and paves the way for scalable, high performance quantum devices based on orientation-controlled NV− centers.
The electronic spin associated with the nitrogen vacancy (NV) color center in diamond is an excellent candidate for a solid-state qubit functioning as a quantum register or sensor. However, the lack of thin film technologies for crystalline diamond with low impurity levels hampers the development of photonic interfaces to such diamond-based qubits. We present a method for manufacturing slabs of diamond of 200 nm thickness and several microns in extent from high-purity single crystal chemical vapor deposition diamond. We measure spin coherence times approaching 100 μ s and observe increased photoluminescence collection from shallow implant NV centers in these slabs. We anticipate these slabs to be appealing as quantum memory nodes in hybrid diamond nanophotonic systems.